So I started by drawing a rectangle that included in the width the knee wall spaces on both sides
I used the Push/Pull tool to reach the ridge height. I knew that the slope of the roof was 12:12 and I could verify that on site. I activated the Protractor which I aligned with a vertical face placing it over one corner. I aligned it with the bottom line, and then I rotated. I typed 12:12 that in this particular case is the same as 45 degrees. I repeated the same steps on the opposite corner to get both guide lines.
I used Push tool to get rid of the outside area where I had no dormers, turned off the X-ray mode and this is what I got.
Since my perspective would be shown from the same place where I took my picture I erase the front wall face instead of using the Section tool to be able to see inside. On the right side I had a roof coming up from a knee wall that formed an L shape with the actual space, so I drew a box on that side. I completed the lines to match the ridge of the main space and the remaining walls.
I turned off the X-ray mode and then I used Position Camera tool to get the perspective view that I wanted, and save a scene. I placed windows and doors allowing letting the sunshine in to get a more dramatic scene. I placed furniture, moldings, textures and a few lighting fixtures to get my first rendering attempt using a pencil style.